meson: add a expat subproject

For Windows

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

meson.build

@@ -1158,7 +1158,7 @@ if dep_thread.found() and host_machine.system() != 'windows'
     pre_args += '-DPTHREAD_SETAFFINITY_IN_NP_HEADER'
   endif
 endif
-dep_expat = dependency('expat')
+dep_expat = dependency('expat', fallback : ['expat', 'expat_dep'])
 # this only exists on linux so either this is linux and it will be found, or
 # it's not linux and wont
 dep_m = cc.find_library('m', required : false)